Talk about roof damage!!! We had a hail storm in this area a few months back that really wreaked havoc on the area roofs.
So for the past couple of months we've been hearing shovels tearing off shingles and nail guns nailing down the new ones.
You are so right about the damage that can be under those shingles too because of water damage. In that case the plywood will have to be replaced and maybe some boards before the new shingles can be put on.
And to do any replacement under the shingles will add quite a bit to the roof job.